Economical hydrocarbon recovery from reservoirs may sometimes prove difficult due to, for example, low recovery efficiencies. To improve efficiencies, enhanced oil recovery processes are typically utilized to increase the amount of hydrocarbon production from a subterranean reservoir. For example, carbon dioxide is sometimes injected into a subterranean reservoir in an attempt to displace the hydrocarbons. Hydrocarbon displacement is primarily achieved through mechanisms including oil swelling and viscosity reduction. In particular, carbon dioxide is miscible with the lighter components of the hydrocarbons such that as they mix, the composition or phase behavior of the hydrocarbons is altered, thus improving the recovery of the hydrocarbons.
FIG. 1 illustrates prior art carbon dioxide flooding of subterranean reservoir 13. Injection well 11 is illustrated extending to a portion of subterranean reservoir 13 that contains hydrocarbons for production. Injection well 11 is in fluid communication with subterranean reservoir 13 and the hydrocarbons therewithin. Production well 15 is positioned a predetermined distance away from injection well 11 and is also in fluid communication with subterranean reservoir 13 in order to receive the hydrocarbons therefrom. As will be readily appreciated by those skilled in the art, there can be additional production wells 15 spaced apart from injection well 11 at predetermined locations to optimally receive the hydrocarbons being pushed through subterranean reservoir due to injections from injection well 11. Injection of carbon dioxide 17 may result in a phenomenon called fingering or channeling in which injected carbon dioxide 17 preferentially follows certain narrow paths 19 through the reservoir formation. This fingering or channeling is often more pronounced in low permeability reservoirs. And unfortunately, this non-uniform spreading results in carbon dioxide 17 bypassing substantial amounts of hydrocarbons in subterranean reservoir 13 such that the bypassed hydrocarbons are not mobilized for recovery.
In an attempt to reduce fingering, water may be injected with the carbon dioxide via the injection wellbore. As described in SPE 113370, the recovery of hydrocarbons may involve different recovery methods. In a WAG process, aqueous brine solutions are injected which are then followed by CO2 injection. Similarly, in a SAG process, aqueous surfactant solutions are injected which are then followed by CO2 injection. And in a modified WAG process, aqueous brine solutions are injected which are then followed by CO2 with dissolved surfactant injection.
While such WAG and SAG processes may improve recovery efficiency in some cases, they may not be able to be used on some reservoirs due to adverse conditions like, for example, extreme temperatures or very low permeability. And even in the cases where such processes can be used, they often lead to other problems. For example, the water and carbon dioxide mixture can form hydrates in the wellbore, which can be problematic by inhibiting or interrupting flow—particularly at high velocities. In addition, at wellbore conditions the CO2 and water are acidic and therefore, may be corrosive to the wellbore and other metals with which they come in contact within the system.
Accordingly, what is needed is an alternative process for economical hydrocarbon recovery especially deepwater applications. It would be beneficial if such a process was useful for low permeability reservoirs. It would further be beneficial if such a process resulted in reduced wellbore corrosion and hydrate formation, reduced reservoir fingering, and enhanced carbon dioxide sweep efficiency compared to conventional methods.
Fortunately, the present inventors have discovered new processes and compositions which may meet one or more of the aforementioned needs or even have other advantages. In one embodiment, the invention relates to a method for enhancing hydrocarbon recovery in low permeability reservoirs. The method comprises first providing a subterranean reservoir comprising one or more hydrocarbons and water therewithin and a wellbore in fluid communication with the subterranean reservoir. Next, carbon dioxide is injected into the reservoir through the wellbore under conditions sufficient to substantially reduce or eliminate water in and near the wellbore. Then a substantially anhydrous composition is injected into the reservoir through the wellbore. The substantially anhydrous composition comprises carbon dioxide and one or more surfactants. The composition is suitable to form a foam upon contact with water within the subterranean reservoir. Advantageously, the method often results in reduced wellbore corrosion, hydrate formation, reduced reservoir fingering, and enhanced carbon dioxide sweep efficiency over conventional methods.
In another embodiment, the invention relates to a method for enhancing hydrocarbon recovery in low permeability reservoirs. The method comprises first providing a subterranean reservoir comprising one or more hydrocarbons and water therewithin, wellbore in fluid communication with the subterranean reservoir, and dehydrated carbon dioxide. Next, the dehydrated carbon dioxide is injected into the reservoir through the wellbore to substantially reduce or eliminate water in and near the wellbore. One or more surfactants are added to the dehydrated carbon dioxide after a predetermined amount of dehydrated carbon dioxide has been injected. For example, the surfactants can be added after approximately 1% to 2% hydrocarbon pore volumes of dehydrated carbon dioxide have been injected. Adding the one or more surfactants to the dehydrated carbon dioxide forms a substantially anhydrous composition that is suitable to form a foam upon contact with water within the subterranean reservoir.